Add a utility function get_registers(frame, kind) to get the registers of a given frame and of a given kind.
Also add three convenience functions get_GPRs(frame), get_FPRs(frame), and get_ESRs(frame) to get the general
purpose registers, the floating point registers, and the exception state registers.

Add TestRegistersIterator.py to test these added functions of lldbutil.py.

llvm-svn: 131144